The present invention relates to medical and surgical imaging, and in particular to intraoperative or perioperative imaging in which images are formed of a region of the patient""s body and a surgical tool or instrument is applied thereto, and the images aid in an ongoing procedure. It is of a special utility in surgical procedures such as brain surgery and arthroscopic procedures on the knee, wrist, shoulder or spine, as well as certain types of angiography, cardiac procedures, interventional radiology and biopsies in which x-ray images may be taken to display, correct the position of, or otherwise navigate a tool or instrument involved in the procedure.
Several areas of surgery have required very precise planning and control for the placement of an elongated probe or other article in tissue or bone that is internal or difficult to view directly. In particular, for brain surgery, stereotactic frames to define the entry point, probe angle and probe depth are used to access a site in the brain, generally in conjunction with previously compiled three-dimensional diagnostic images such as MRI, PET or CT scan images which provide accurate tissue images. For placement of pedicle screws in the spine, where visual and fluoroscopic imaging directions cannot capture an axial view necessary to center the profile of an insertion path in bone, such systems have also been useful.
When used with existing CT, PET or MRI image sets, these previously recorded diagnostic image sets themselves define a three dimensional rectilinear coordinate system, by virtue of their precision scan formation or the spatial mathematics of their reconstruction algorithms. However, it may be necessary to correlate the available fluoroscopic views and anatomical features visible from the surface or in fluoroscopic images with features in the 3-D diagnostic images and with the external coordinates of the tools being employed. This is often done by providing implanted fiducials, and adding externally visible or trackable markers that may be imaged, and using a keyboard or mouse to identify fiducials in the various images, and thus identify common sets of coordinate registration points in the different images, that may also be trackable in an automated way by an external coordinate measurement device, such as a suitably programmed off-the-shelf optical tracking assembly. Instead of imageable fiducials, which may for example be imaged in both fluoroscopic and MRI or CT images, such systems can also operate to a large extent with simple optical tracking of the surgical tool, and may employ an initialization protocol wherein the surgeon touches or points at a number of bony prominences or other recognizable anatomic features in order to define the external coordinates in relation to the patient anatomy and to initiate software tracking of those features.
Generally, systems of this type operate with an image display which is positioned in the surgeon""s field of view, and which displays a few panels such as a selected MRI image and several x-ray or fluoroscopic views taken from different angles. The three-dimensional diagnostic images typically have a spatial resolution that is both rectilinear and accurate to within a very small tolerance, e.g., to within one millimeter or less. The fluoroscopic views by contrast are distorted, and they are shadowgraphic in that they represent the density of all tissue through which the conical x-ray beam has passed. In tool navigation systems of this type, the display visible to the surgeon may show an image of the surgical tool, biopsy instrument, pedicle screw, probe or the like projected onto a fluoroscopic image, so that the surgeon may visualize the orientation of the surgical instrument in relation to the imaged patient anatomy, while an appropriate reconstructed CT or MRI image, which may correspond to the tracked coordinates of the probe tip, is also displayed.
Among the systems which have been proposed for effecting such displays, many rely on closely tracking the position and orientation of the surgical instrument in external coordinates. The various sets of coordinates may be defined by robotic mechanical links and encoders, or more usually, are defined by a fixed patient support, two or more receivers such as video cameras which may be fixed to the support, and a plurality of signaling elements attached to a guide or frame on the surgical instrument that enable the position and orientation of the tool with respect to the patient support and camera frame to be automatically determined by triangulation, so that various transformations between respective coordinates may be computed. Three-dimensional tracking systems employing two video cameras and a plurality of emitters or other position signaling elements have long been commercially available and are readily adapted to such operating room systems. Similar systems may also determine external position coordinates using commercially available acoustic ranging systems in which three or more acoustic emitters are actuated and their sounds detected at plural receivers to determine their relative distances from the detecting assemblies, and thus define by simple triangulation the position and orientation of the frames or supports on which the emitters are mounted. When tracked fiducials appear in the diagnostic images, it is possible to define a transformation between operating room coordinates and the coordinates of the image.
In general, the feasibility or utility of a system of this type depends on a number of factors such as cost, accuracy, dependability, ease of use, speed of operation and the like. Intraoperative x-ray images taken by C-arm fluoroscopes alone have both a high degree of distortion and a low degree of repeatability, due largely to deformations of the basic source and camera assembly, and to intrinsic variability of positioning and image distortion properties of the camera. In an intraoperative sterile field, such devices must be draped, which may impair optical or acoustic signal paths of the signal elements they employ to track the patient, tool or camera.
More recently, a number of systems have been proposed in which the accuracy of the 3-D diagnostic data image sets is exploited to enhance accuracy of operating room images, by matching these 3-D images to patterns appearing in intraoperative fluoroscope images. These systems may require tracking and matching edge profiles of bones, morphologically deforming one image onto another to determine a coordinate transform, or other correlation process. The procedure of correlating the lesser quality and non-planar fluoroscopic images with planes in the 3-D image data sets may be time-consuming, and in those techniques that rely on fiducials or added markers, the processing necessary to identify and correlate markers between various sets of images may require the surgeon to follow a lengthy initialization protocol, or may be a slow and computationally intensive procedure. All of these factors have affected the speed and utility of intraoperative image guidance or navigation systems.
Correlation of patient anatomy or intraoperative fluoroscopic images with precompiled 3-D diagnostic image data sets may also be complicated by intervening movement of the imaged structures, particularly soft tissue structures, between the times of original imaging and the intraoperative procedure. Thus, transformations between three or more coordinate systems for two sets of images and the physical coordinates in the operating room may require a large number of registration points to provide an effective correlation. For spinal tracking to position pedicle screws it may be necessary to initialize the tracking assembly on ten or more points on a single vertebra to achieve suitable accuracy. In cases where a growing tumor or evolving condition actually changes the tissue dimension or position between imaging sessions, further confounding factors may appear.
When the purpose of image guided tracking is to define an operation on a rigid or bony structure near the surface, as is the case in placing pedicle screws in the spine, the registration may alternatively be effected without ongoing reference to tracking images, by using a computer modeling procedure in which a tool tip is touched to and initialized on each of several bony prominences to establish their coordinates and disposition, after which movement of the spine as a whole is modeled by optically initially registering and then tracking the tool in relation to the position of those prominences, while mechanically modeling a virtual representation of the spine with a tracking element or frame attached to the spine. Such a procedure dispenses with the time-consuming and computationally intensive correlation of different image sets from different sources, and, by substituting optical tracking of points, may eliminate or reduce the number of x-ray exposures required to effectively determine the tool position in relation to the patient anatomy with the required degree of precision.
However, each of the foregoing approaches, correlating high quality image data sets with more distorted shadowgraphic projection images and using tracking data to show tool position, or fixing a finite set of points on a dynamic anatomical model on which extrinsically detected tool coordinates are superimposed, results in a process whereby machine calculations produce either a synthetic image or select an existing data base diagnostic plane to guide the surgeon in relation to current tool position. While various jigs and proprietary subassemblies have been devised to make each individual coordinate sensing or image handling system easier to use or reasonably reliable, the field remains unnecessarily complex. Not only do systems often require correlation of diverse sets of images and extensive point-by-point initialization of the operating, tracking and image space coordinates or features, but they are subject to constraints due to the proprietary restrictions of diverse hardware manufacturers, the physical limitations imposed by tracking systems and the complex programming task of interfacing with many different image sources in addition to determining their scale, orientation, and relationship to other images and coordinates of the system.
Several proposals have been made that fluoroscope images be corrected to enhance their accuracy. This is a complex undertaking, since the nature of the fluoroscope""s 3D to 2D projective imaging results in loss of a great deal of information in each shot, so the reverse transformation is highly underdetermined. Changes in imaging parameters due to camera and source position and orientation that occur with each shot further complicate the problem. This area has been addressed to some extent by one manufacturer which has provided a more rigid and isocentric C-arm structure. The added positional precision of that imaging system offers the prospect that, by taking a large set of fluoroscopic shots of an immobilized patient composed under determined conditions, one may be able to undertake some form of planar image reconstruction. However, this appears to be computationally very expensive, and the current state of the art suggests that while it may be possible to produce corrected fluoroscopic image data sets with somewhat less costly equipment than that required for conventional CT imaging, intra-operative fluoroscopic image guidance will continue to require access to MRI, PET or CT data sets, and to rely on extensive surgical input and set-up for tracking systems that allow position or image correlations to be performed.
Thus, it remains highly desirable to utilize simple, low-dose and low cost fluoroscope images for surgical guidance, yet also to achieve enhanced accuracy for critical tool positioning.
It would be desirable to provide an improved image guided navigation system for a surgical instrument.
It would also be desirable to provide such an image guided system which operates with a C-arm fluoroscope to produce enhanced images and information.
It would also be desirable to provide an image-guided surgical navigation system adaptable to a fluoroscope that accurately depicts tool position.
One or more of the foregoing features and other desirable ends are achieved in a method or system of the present invention wherein an x-ray imaging machine of movable angulation, such as a fluoroscope, is operated to form reference or navigation images of a patient undergoing a procedure. A tracking system employs a tracking element affixed to each of the imaging machine and tool, and preferably to the patient as well, to provide respective position data for the tool, the fluoroscope and patient, while a fixed volume array of markers, which is also tracked, is imaged in each frame. Preferably the array of markers is affixed to the detector assembly of the imaging machine, where a single tracking element determines position of the fluoroscope and entire array of markers. The fluoroscope may itself also provide further shot-specific indexing or identification data of conventional type, such as time, settings or the like. A processor then applies the position data from the tracking system, and operates on the imaged markers to produce a correct tool navigation image for surgical guidance.
The markers are preferably arranged in a known pattern of substantially non-shadowing point elements positioned in different planes. These may be rigidly spaced apart in a predefined configuration in an assembly attached to the fluoroscope, so that the physical position of each marker is known exactly in a fixed fluoroscope-based coordinate system, and the positions may, for example, be stored in a table. A single tracking element may be affixed on the marker assembly, which may in turn be locked in a fixed position on the fluoroscope, so that the fluoroscope and marker positions are known in relation to the tool and the patient. Alternatively, one or more separate arrays of markers may be independently positioned and each tracked by a separate tracking element.
In each fluoroscopic image, the processor identifies a subset of the markers and recovers geometric camera calibration parameters from the imaged marker positions. These calibration parameters then allow accurate reference between the recorded image and the tool and patient coordinates measured by the trackers. The processor may also receive patient identification data of a conventional type to display or record with the shot. In one embodiment the processor computes the calibration as well as geometric distortion due to the imaging process, and converts the tracked or actual location of the tool to a distorted tool image position at which the display projects a representation of the tool onto the fluoroscopic image to guide the surgeon in tool navigation.
In this aspect of the invention, the processor identifies markers in the image, and employs the geometry of the identified markers to model the effective source and camera projection geometry each time a shot is taken, e.g., to effectively define its focus and imaging characteristics for each frame. These parameters are then used to compute the projection of the tool in the fluoroscope image.
In yet a further aspect of the invention, the fluoroscope is operated to take a series of shots in progressively varying orientations and positions as the camera and source are moved about the patient. Accurate calibration for multiple images is then employed to allow three-dimensional reconstruction of the image data. The processor applies a reconstruction operation or procedure, for example, back projection of the registered images to form a volume image data set, e.g., a three dimensional set of image density values of a tissue volume. The initial set of fluoroscopic images may, for example, be acquired by taking a series of views rotating the fluoroscope in a fixed plane about a target region of tissue. A common center and coordinate axes are determined for the reconstructed volume, such that the volume image data set constructed from the images corresponds to the target region. Image planes are then directly constructed and displayed from this volume image data set.
The resultant fluoro-CT images are geometrically comparable to conventional diagnostic image sets of the imaged volume, and obviate the need for complex tracking and image correlation systems otherwise proposed or required for operating-room management and display of pre-operatively acquired volumetric data sets with intraoperative fluoro images. In accordance with a still further aspect of the invention, this reconstructed fluoro-CT data set is then registered to or transformed to the image space coordinates of a preoperative PET, MRI or CT data set for simultaneous display of both sets of images. In other embodiments, the system of the present invention may be used simply for the purpose of intraoperatively registering preoperative 3D image data to the patient tissue. In accordance with this aspect of the invention, a set of fluoro-CT image data is constructed as described above, and these are registered to preoperative 3D image data by mutual information, contour matching or other correlation procedure. This provides a direct registration of the preoperative data to tracking coordinates without requiring the surgeon to place and image fiducials, touch and enter skeletal or surface registration points, or perform invasive pre-operation image registration protocols.
The tracking elements of the tracking system may comprise various position-indicating elements or markers which operate optically, ultrasonically, electromagnetically or otherwise, and the tracking system itself may include hybrid software-mediated elements or steps wherein a pointer or tool of defined geometry is tracked as it touches fiducials or markers in order to enter or initialize position coordinates in a tracking system that operates by triangulating paths, angles or distances to various signal emitting or reflecting markers. A hybrid tracking system may also be used, including one or more robotic elements which physically encode mechanical positions of linkages or supports as part of one or more of the tracking measurements being made. Preferably, however, the tracking system employs electromagnetic tracking elements such as shown in U.S. Pat. No. 5,967,980, to generate and/or detect electromagnetic field components that pass through or are substantially unobstructed by the patient and intervening structures, and to directly determine coordinates in three or more dimensions referenced to the tool, the patient or the fluoroscope to which the elements are attached.
A single tracking element may be affixed to each of the fluoroscope, the patient, and the surgical tool. One presently preferred embodiment of a tracking element employs a magnetic field element, such as one configured with three mutually orthogonal coils, that otherwise operates as a substantially point-origin field generator or field sensor. The element may have a rigid or oriented housing, so that when attached to a rigid object, the tracked coordinates of the element yield all coordinates, with only a defined constant offset, of the object itself. The element may be energized as a field generator, or sampled as a field sensor, to produce or detect a field modulated in phase, frequency or time so that some or all of the x-, y-, z-, roll-, pitch-, and yaw coordinates of each tracking element, and thus its associated object, are quickly and accurately determined. A table of position correction factors or characteristics may be compiled for one or more of the tracking elements to correct for the effects of electromagnetic shunting or other forms of interference with the generator or receiver which may occur when positioned in a region near to the body of the fluoroscope. This allows a magnetic tracking element to be placed quite close to the imaging assembly or other conductive structure and achieve high position tracking accuracy or resolution. In particular, one or more tracking elements may be mounted directly on the fluoroscope and/or on calibration fixtures positioned close to the image detector of the fluoroscope to define camera and imaging parameters relative to another tracker which may move with the patient or with a tool. Various alternative magnetic generating and sensing assemblies may be used for the tracking component, such as ones having a tetrahedrally-disposed generating element and a single sensing/receiving coil, or ones having a multipole generating assembly that defines a suitably detectable spatial field.